1. Field of the Invention
This invention relates to hydrocyclone cleaners used for cleaning a suspension of papermakers' fibers, otherwise commonly known as papermakers' stock, and more particularly to a reverse cleaning hydrocyclone with extended dwell time.
2. Description of the Related Art
The terms "forward cleaning" and "reverse cleaning" have become well understood in the art of cleaning papermakers' stock, and relate primarily to the manner in which a cyclone-type centrifugal cleaner is operated. Examples of cyclone-type cleaners connected and used as forward cleaners, in which the accepts are removed at the base of the cone while the rejects are removed from the apex, are shown in Samson et al, U.S. Pat. No. 2,377,524 issued Jun. 5, 1945 and Grundelius et al, U.S. Pat. No. 3,486,619 issued Dec. 30, 1969. In a reverse cleaner system, a cyclone-type cleaner is operated in such a manner that the accepts are removed from the apex of the cone, while the lighter rejects are taken out at the base, as shown in Braun, U.S. Pat. No. 3,912,579 issued Oct. 14, 1975, and in Braun et al, U.S. Pat. No. 3,557,956 issued Jan. 26, 1971.
Hydrocyclone cleaners have been used for many years in the preparation of papermakers' stock and particularly for cleaning such stock by removing undesirable heavier and lighter weight components from the water base. Until the advent of the teachings of Braun as disclosed in U.S. Pat. No. 3,912,579, such hydrocyclone cleaners were operated in what was then considered the "conventional" manner, in which the heavier contaminants were extracted from a bottom or apex outlet, while the "good" fibers and water absent the heavier contaminants were extracted from a top outlet in the base of the cone.
Flow conditions within the hydrocyclone have been studied and reported in the patent literature, and two patents stand out by reason of detailed analysis of the hydrocyclone and attempts to arrive at optimum diameters and the optimum taper or slope of the cone walls. These include Samson et al. U.S. Pat. No. 2,377,524 and Tomlinson U.S. Pat. No. 3,096,275.
Samson et al. proposed a considerably longer cone length than had herebefore been used, and recommended a cone length-to-base diameter ratios of about 11 to 1 (cone diameter at base 3", length of cone 33") page 4, column 2, lines 12-15, up to about 15 to 1 (page 5 col. 2 lines 30-50). The latter example provided an included cone angle, of about 3.6 degrees.
Since the heavy material which is to be separated flows to the outside of the vortex and alongside the inside sloping surfaces of the cone, too great a cone angle itself will provide resistance to the separation of the heavier components, since a steep cone angle will resist the flow of these heavier components to the apex outlet. In spite of the use of the shallow cone angles, Samson et al. did not have particularly good success since they lost between 20% and 33% of the dry weight of the good fibers through the bottom rejects opening along with the heavy contaminants (page 3 col. 2 lines 15-30, and page 5 col. 1, lines 29-51). It is likely that this high loss of good fiber was, at least in part, due to the fact that the good papermakers' fibers also have a specific gravity which is greater than water and tend also to be carried to the wall of the hydrocyclone and then become separated with the heavier contaminants.
Tomlinson U.S. Pat. No. 3,096,275 re-studied the problem of defining the efficient size and shape of a hydrocyclone and, in his Example 13, Table III, he even tested a unit having the measurements "selected by Samson and Croup" (col. 12, lines 24-33). He recommended the use of large diameter cones with a head section of from 7" to 12" in diameter (column 4 lines 38-45) and he increased the included angle to between 10.degree. and 18.degree.. As a result, Tomlinson was able to control the discharge from the rejects outlet to a very small flow where it contained only 1.310% of the total solids applied, (column 4 line 73). By use of a large cone and a steep wall angle, he obtained higher internal shear to which he attributed the high rate of retention of good fibers (column 7 lines 55-75).
When Braun, and others, began operating the hydrocyclone as a "reverse cleaner" the hydrocyclone construction remained essentially in the shape preferred by Tomlinson. This arrangement was effective in removing light weight rejects since it was no longer necessary to force a differentiation between the heavier "good" paper fibers and heavy contaminants. However, no effort is known to have been made to optimize the shape of a hydrocyclone specifically for reverse cleaning, such as by lengthening the cone, since Tomlinson had shown that higher included angles provided a better separating mechanism. There is accordingly a need for a reverse hydrocyclone specifically designed for more efficiently separating lightweight contaminants from papermakers' stock.